1. Field of the Invention
The present invention relates to a technology for determining an intensity of a pumping light having a plurality of wavelengths to realize a desired gain profile in a Raman amplifier capable of amplifying a broadband signal light.
2. Description of the Related Art
Optical amplifiers are indispensable for realizing a long-distance and large-capacity optical communication system. The optical amplifiers are also effective as a means for compensating for a decrease in light signal intensity in a metro/access system, and are widely used in various optical communication systems.
One of the representative optical amplifiers is a rare-earth-doped optical fiber amplifier such as an erbium-doped fiber amplifier (EDFA) that is mainly applied to a signal band of 1.55 micrometer that is an amplification band of the EDFA. However, in recent years, since a transmission capacity required of an optical communication system has been increasing rapidly, a significant increase in a signal band is required. Therefore, it is difficult to secure a sufficient signal band only with the rare-earth-doped optical fiber amplifier. An optical amplifier applicable to optical signals in a wider band is demanded.
A Raman amplifier makes use of stimulated Raman scattering that is caused by pumping light lead into a silica fiber. Therefore, it is possible to set an amplification wavelength freely by changing a pumping light wavelength and realize a desired gain wavelength characteristic by adjusting an output distribution of pumping light consisting of a plurality of wavelengths. These are characteristics advantageous for an optical amplifier used for broadband wavelength multiplexing transmission. The rare-earth-doped optical fiber amplifier does not have the characteristics.
Various technologies utilizing these characteristics of the Raman amplifier have been proposed. The technologies include a technology for adjusting a gain wavelength characteristic by changing a pumping light output consisting of a plurality of wavelengths arbitrarily (U.S. Pat. No. 6,115,174) and a technology for realizing a flat gain wavelength characteristic by setting a plurality of pumping wavelength intervals properly (U.S. Pat. No. 6,292,288).
On the other hand, to put the Raman amplifier into practice widely, it is necessary to obtain appropriate pumping light outputs automatically and promptly such that a desired gain wavelength characteristic is obtained. However, it is impossible to determine pumping light outputs for realizing a desired gain wavelength characteristic with the technologies described above. This is because an inverse problem has to be solved to determined pumping light power and, if a nonlinear optimization method is used to solve the inverse problem, convergence of solutions depends on initial values. A complicated optimization algorithm is required to cause solutions to converge on an optimum value with respect to initial values in a wider range.
As an example of such an algorithm, there is a technology that neglects inter-signal Raman and adopts an automatic determination method for pumping wavelength and power using a generic algorithm (Victor E. Perlin, et al., (2002), “Efficient design method for multi-pump flat-gain fiber Raman amplifiers”, OFC2002, TuJ1, p. 57). Note that, as a technology for determining a pumping light output more easily, there is a technology for storing optical outputs of pumping wavelengths, which realize an expected gain wavelength profile, in a memory and extracting a condition close to a desired profile from the memory to perform control (U.S. Publication No. 2001/0050802 A1). There is also a literature that describes a control method for preventing fluctuation of a transitional pumping state due to an increase or a decrease of pumping light sources in expanding or reducing an amplification wavelength band (Japanese Patent Application No. 2001-109361).